This application claims the priority of German Patent No. 19918513.1, filed Apr. 23, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an electric drive arrangement for internal combustion engines in motor vehicles having an electric starter coupled to the internal combustion engine, and an electric generator drive connected with the internal combustion engine.
For decades, internal combustion engines in road vehicles have been started by means of an electronic starter which is connected mechanically to the crankshaft of the internal combustion engine via its starter pinion and the ring gear on the flywheel only during the starting phase. Nowadays, all starter motors have the torque characteristic of a series-wound machine, which is characterized essentially by an output torque which falls continuously to zero as the speed increases, beginning with the maximum possible torque at standstill. During the starting operation this characteristic leads to a torque equilibrium between the starter torque and the drag torque of the internal combustion engine at crankshaft speeds of between 80 and 200 rpm, which are sufficient for starting according to the requirements which have applied hitherto. To make these starter motors as small, light and cheap as possible, d.c. machines of series-wound construction or with permanent-magnetic excitation and as large as possible a transmission ratio relative to the crankshaft have been used. A total transmission ratio of the intermediate gear in the starter and the transmission ratio of the starter pinion relative to the crankshaft is about 60:1. This high transmission ratio necessitates an engagement device which establishes a driving connection with the crankshaft only when the starter is actuated and hence protects the starter motor from extreme speeds.
Future exhaust regulations will make the design of the above-described starting system inadequate. The minimum starting speeds hitherto stipulated at the crankshaft lead to high pollutant emissions in the current starting operation and it is known that there is considerable potential for improvement in this area by raising the starting speed to values within the range of the idling speed of the internal combustion engine.
Another disadvantage of current starting systems is that starting operations last a relatively long time and are relatively loud due to the high transmission ratio of the starter relative to the crankshaft and the necessary engagement and disengagement of the starter pinion. This is becoming less and less acceptable to the customer, especially as future operating concepts for vehicles aimed at achieving fleet consumption targets will require a significantly larger number of starting operations. This will result in problems with the life of current starting systems.
European Patent 0 793 013 A1 discloses a starting operation using a belt-driven generator instead of a conventional starter. Problems arise at very low temperatures since the maximum torque of such an arrangement is no longer sufficient to start the engine reliably under such circumstances. Therefore such systems are restricted to internal combustion engines with small displacements, at best. If a design for large internal combustion engines were implemented which also operated reliably at low temperatures, this would lead to unacceptable large, heavy and expensive generators.
European Patent 0 406 182 B1 attempts to solve this problem by using a boost circuit to generate a higher voltage for starting, briefly bringing about higher currents in the generatorxe2x80x94operated as a motorxe2x80x94and hence increasing torque. However, this solution has the disadvantage that the belt drive loses adhesion at low temperatures and tends to slip. In addition, the charging time of the starting energy storage is disruptive, thereby rendering it impossible to perform starting operations is rapid succession. The life of such an arrangement is furthermore inadequate for the future concepts, which necessitate a large number of starting operations.
One object of the present invention is an economical electric drive arrangement for internal combustion engines which provides more rapid and more comfortable starting operation, and which also takes place reliably at low temperatures.
In particular, the drive arrangement according to the invention has the advantage that, in combined operation, the driving torques of the generator and the starter are superimposed in an effective manner since the torque of the active generator cuts in precisely when the starter begins to slacken off. The crankshaft of the internal combustion engine is thereby accelerated very rapidly to well beyond the speed that can currently be achieved by a starter, thus ensuring rapid and reliable starting even at low temperatures. In the case of a cold start, a division of tasks occurs between the conventional starter (overcoming the break-away torque) and the generator (increasing the cranking torque in the range of higher crankshaft speeds). Over dimensioning of the generator together with the power electronics and an unwanted intervention in the drive line can be avoided, and a starter, can be used virtually unaltered. The solution according to the invention thus represents an optimum cost solution. At higher outside temperatures and/or with a warm engine, the starting operation can be performed solely with the generator, thereby enabling particularly comfortable starting with particularly little noise. The electronic control device advantageously decides the starting operation mode as a function of at least on temperature sensor.
The semiconductor circuit to accomplish the present invention can advantageously be connected as an inverter for motor operation of the generator, which is designed as an a.c. or d.c. generator, and as a rectifier for generator operation, making it possible to use a single semiconductor circuit for both modes of operation, with a corresponding improvement in efficiency by active rectification in the generator mode.
The decision as to whether the starting operation should be performed solely by means of the generator operated as a motor or by means of this generator in combination with the starter is made in an optimum manner by the electronic control device as a function of the engine-oil temperature and/or the outside temperature and/or the off time of the internal combustion engine. To start the internal combustion engine at low temperature, the generator and the starter can be switched on simultaneously or in succession with or without a time overlap. These variants can, for example, be chosen as alternatives depending on the respective starting parameters.
It is further advantageous that the electronic control device be designed to set a defined angular position of the crankshaft of the internal combustion engine when switching the engine off with the aid of the generator, which is operated as a motor. For this purpose, the generator is, for example, designed as a fully functional four-quadrant positioning drive. This allows the next starting operation to take place from a defined initial position, thereby considerably speeding up the starting operation and considerably reducing pollutant emissions. Moreover, one of the hitherto customary sensors, either the TDC (Top Dead Center) sensor or the camshaft sensor can be omitted. Activating the ignition and/or injection during the starting operation of the internal combustion engine only at speeds close to the starting speed, e.g. at 80% of the starting speed is particularly favourable for low energy consumption and low pollutant emissions.
The electronic control device can also advantageously be used to support deceleration operations of the internal combustion engine by switching on the generator, operated in the generator mode, and/or to assist acceleration operations of the internal combustion engine by switching on the generator, which operated in the motor mode. For example, operation of the generator as a motor can be used to assist driving dynamics while the internal combustion engine is running, i.e. as an intervention of the active generator for the purpose of assistance during all acceleration processes of the internal combustion engine. On the other hand, it is also possible for rotational energy of the internal combustion engine to be recovered when the generator is operated as a generator, especially in overrun and braking mode, with the result that the generator additionally assists the desired deceleration process. This mode selection takes place most efficiently as a function of the speed of the internal combustion engine.
The active generator can also advantageously be involved in synchronizing the engine and gearbox speeds during gear changes in manual gearboxes. The energy which has to be converted in the synchronizer rings of the gearbox to equalize the rotational speeds can be recovered by means of the generator, and, in addition, more rapid gear changes are obtained, possibly even eliminating the need for synchronizer rings and a clutch, given appropriate design. During a prospective gear change, the engine is controlled in such a way with the assistance of the generator that no torque is transmitted in the drive line. The respective gear can then be disengaged. Re-equalization of the speed then takes place with the aid of an electronic accelerator or with the aid of an electronic throttle valve and electronic assistance. When the engine speed has been synchronized with the new gearbox speed, no jerking movement occurs during the new gear engagement.
It is also possible to prevent excessive device belt slip between the internal combustion engine and the generator by switching on generator operation of the generator or motor operation of the generator. Additional torques at the belt drive can thereby be connected and disconnected or compensated for in a manner which spares the belt, thus increasing the life of the drive belt and largely preventing defects.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.